The spunbonding process is widely used for producing nonwoven fabrics from thermoplastic filaments. Spunbonded fabrics can be produced by many routes, but the majority of spunbonding processes include the basic steps of extruding continuous filaments of a fiber-forming thermoplastic polymer, quenching the filaments, drawing or attenuating the filaments, usually by a high velocity fluid, and depositing the filaments on a collection surface to form a web.
Manufacturers of spunbonded nonwoven fabrics have long sought to improve the manufacturing process to achieve higher productivity and better quality and uniformity of the spunbonded nonwoven fabric. Maintaining the quality and uniformity of the fabric becomes a particular concern at higher production speeds and when producing fabrics of low basis weight. Several characteristics affect the quality and uniformity of spunbonded nonwoven fabrics.
Filament separation is the degree of separation of the individual filaments from one another. Good filament separation occurs when the filaments are randomly arranged with limited parallel contact between the filaments. Ideally, no individual filaments should be in parallel contact with another filament, although, in practice, filaments tend to be in parallel contact over considerable distances. Good filament separation is particularly important for light weight fabrics, where good coverage is more difficult to achieve. Ropiness is the extreme state of poor filament separation. Large numbers of filaments in parallel twisted contact result in long strands in the fabric, which can causes holes or very thin areas in the fabric. Splotchiness is a relative large-scale non-uniformity in basis weight. A fabric having splotchiness is generally weak because of the lower tensile strength of the thin areas of the fabric. Also, a splotchy fabric generally has poor cover properties.
In the early spunbond processes which used round attenuator tubes to attenuate and draw the filaments, achieving good uniformity and adequate cover presented significant challenges, particularly when manufacturers attempted to produce lighter weight webs or to produce webs at higher speeds or reduced cost. The round attenuator tubes, often called Lurgi tubes, typically use large quantities of high pressure air that provide the attenuation force for the filaments. This results in high utility costs and high noise levels. Increasing the number of filaments in each tube to increase productivity and to reduce the utility expense results in increased problems of poor filament separation, ropiness and webs having poor cover.
Many attempts have been made to overcome the above problems of filament separation, ropiness and splotchiness while still preserving the tensile properties of nonwoven webs made from spunbonded thermoplastic filaments. For example, U.S. Pat. Nos. 3,296,678; 3,485,428 and 4,163,305 describe various apparatus and methods for mechanical and pneumatic oscillation of continuous filament bundles to spread the filaments as they are deposited on the collection surface. U.S. Pat. No. 4,334,340 describes using an air foil at the exit of a round attenuator tube to separate continuous filaments prior to their deposit on a forming wire. Forced air follows the leading edge of the air foil and filaments striking the foil are carried by the forced air onto a forming wire, resulting in a spreading of the filament bundle that promotes random deposit of the filaments.
Various electrostatic methods have been proposed to promote spreading of the filament bundle by applying an electric charge to the filaments to cause the filaments to repel one another. U.S. Pat. No. 3,338,992 describes triboelectric charging, in which the filaments are charged by rubbing contact with a suitable dielectric material and repelling forces induced in the filament bundle cause the filaments to separate as they exit a forwarding gun and prior to deposit on the forming wire. However, rubbing contact typically is not desirable for more delicate webs, and this method is also subject to lack of reliability when ambient conditions change. The above-noted U.S. Pat. Nos. 3,338,992 and 3,296,678 also describe electrostatically charging the filament bundle with an ion gun or corona discharge device prior to drawing and forwarding the filaments.
U.S. Pat. No. 4,208,366 describes a spunbonding process without the use of forced air attenuation, but which includes electrostatic treatment of the filament bundle. The extruded filaments pass through an electrostatic charging zone and are drawn through a nip between elastomer covered draw rolls. The charged filaments are propelled by the draw rolls into an electrostatic field generated between the rolls and the collecting surface, which attracts the filaments to the collecting surface.
U.S. Pat. Nos. 3,163,753, 3,341,394, and 4,009,508 relate to the use of corona electrodes for electrostatic treatment of filament bundles attenuated with round attenuator guns. In U.S. Pat. No. 3,163,753, the filament bundle is passed adjacent a charged corona electrode while passing over a grounded bar. In U.S. Pat. No. 3,341,394, a corona is applied while the filaments are under tension and before the filaments enter the attenuation tube. In U.S. Pat. No. 4,009,508, the filaments are subjected to electrostatic treatment from a corona after they have been discharged from the round attenuator tube and while the filaments impinge upon a target electrode for spreading in the electric field.
Various slot attenuators have been developed to overcome the problems and limitations of the round attenuator. In a slot attenuator, or slot draw process, the multiple tube attenuators are replaced with a single slot-shaped attenuator that covers the full width of the machine. A supply of air is admitted into the slot attenuator below the spinneret face. The air proceeds down the attenuator channel, which narrows in width, creating a venturi effect to accelerate the air flow and cause filament attenuation. The filaments exit the attenuator channel and are collected on the forming wire. The attenuation air, depending on the type of slot draw process used, can be directed into the attenuation slot by a pressurized air supply above the slot, or by a vacuum located below the forming wire. Slot drawing has various advantages over the Lurgi and other tube-shaped attenuator processes. The slot attenuator is self-threading in that the filaments fall out of the spin block directly into the slot attenuator. The high pressure air used by Lurgi devices is not always required, thereby reducing noise and utility costs.
However, despite the advantages of the slot draw process, cover problems can still occur, particularly for lighter weight fabrics. The forced air stream can introduce turbulence at the point where the web is formed on the collection surface, which adversely affects the quality of the web. Additionally, manufacturers are still attempting to produce webs at higher processing speeds, which compounds the problem. For example, U.S. Pat. No. 4,753,698 describes a technique for mechanically oscillating the rank of filaments exiting a slot draw attenuator and applying vacuum through the forming wire to fix the filaments in place. Coanda rolls set up a pendular movement in the filament rank. However, the swinging velocity of the filaments at the reversal points is zero, and, unless special precautions are taken, pile-ups can occur at the reversal points.
In view of the advantages of the slot draw process over prior filament attenuation techniques, it would be desirable to provide a slot-draw process capable of producing spunbonded fabrics having better cover properties. Accordingly, it is an object of the present invention to provide a slot draw process and apparatus for producing a spunbonded nonwoven web having improved cover properties. More particularly, it is an object of the present invention to provide a slot draw process and apparatus capable of producing nonwoven webs having excellent cover characteristics, despite low basis weight or high processing speeds.